大气氮沉降输入会增加森林生态系统氮素有效性,进而改变土壤N_2O产生与排放,然而有关不同氮素离子(氧化态NO_3^--N与还原态NH_4^+-N)沉降对土壤N_2O排放的影响知之甚少。以大兴安岭寒温带针叶林为研究对象,构建了3种类型(NH_4Cl、KNO_3、NH_4NO_3)和4个施氮水平(0、10、20、40 kg N hm^(-2)a^(-1))的增氮控制试验,利用流动化学分析仪和静态箱-气相色谱法4次/月测定凋落物层和矿质层土壤无机氮含量、土壤-大气界面N_2O净交换通量以及相关环境因子,分析施氮类型和剂量对土壤氮素有效性、土壤N_2O通量的影响探讨氮素富集条件下土壤N_2O通量的环境驱动机制。结果表明:施氮类型和剂量均显著影响土壤无机氮含量,土壤NH_4^+-N的积累效应显著高于NO_3^--N。施氮一致增加寒温带针叶林土壤N_2O排放,NH_4NO_3促进效应最为明显,增幅为442%-677%,高于全球平均水平(134%)。土壤N_2O通量与土壤温度、凋落物层NH_4^+-N含量正相关,且随着施氮水平增加而增加。结果表明大气氮沉降短期内不会导致寒温带针叶林土壤NO_3^--N大量流失,但会显著促进土壤N_2O的排放。此外,外源性NH_4^+和NO_3^-输入对土壤N_2O排放的促进作用具有协同效应,在未来森林生态系统氮循环和氮平衡研究中应该区分对待。
Introduction:Soil heterotrophic respiration(Rh,an indicator of soil organic carbon decomposition)is an important carbon efflux of terrestrial ecosystems.However,the dynamics of soil Rh and its empirical relations with climatic factors have not been well understood.Methods:We incubated soils of three subtropical forests at five temperatures(10,17,24,31,and 38°C)and five moistures(20,40,60,80,and 100%water holding capacity(WHC))over 90 days.Rh was measured throughout the course of the incubation.Three types of models(log-linear,exponential,and power model)were fitted to the measurements and evaluated based on the coefficient of determination(r2)and Akaike Information Criterion(AIC)of the model.Further regression analysis was used to derive the empirical relations between model parameters and the two climatic factors.Results:Among the three models,the power function model(Rh=R1 t−k)performed the best in fitting the descending trend of soil Rh with incubation time(r2>0.69 for 26 of 30 models).Both R1 and k generally increased linearly with soil temperature but varied quadratically with soil moisture in the three forest soils.Conclusions:This study demonstrated that the power function model was much more accurate than the exponential decay model in describing the decomposition dynamics of soil organic carbon(SOC)in mineral soils of subtropical forests.The empirical relations and parameter values derived from this incubation study may be incorporated into process-based ecosystem models to simulate Rh responses to climate changes.
